It is known that semipermeable membranes useful in various separations can be made by interfacial polymerization processes conducted on the surfaces of porous substrates. In particular, a variety of semipermeable membranes known in the art as thin-film-composite membranes have been under development since the 1960s, resulting in commercial products now widely used in water purification, brackish water desalting, and potable water production from seawater. Principal among these membranes are aromatic polyamides prepared by interfacial reaction of aqueous aromatic polyamines with nonaqueous polyacyl halides, wherein the interfacial reaction is customarily performed on the surface of porous polysulfone substrates, the latter being usually further supported by a backing of a woven or nonwoven fibrous polyester or polyolefinic web. These are commonly referred to as thin film composite membranes in the art. Most of the current membranes of this type find their origin in methods and compounds originally taught in U.S. Pat. No. 4,277,344, issued to Cadotte, wherein aromatic diamines were interfacially reacted with aromatic polyacyl halides, trimesoyl chloride being the most preferred polyacyl halide. The commercial success of interfacially formed, aromatic polyamide membranes made according to the teachings of Cadotte has resulted in various versions thereafter with equivalent or improved performance. Particularly noteworthy in commerce are membranes with competitive performance characteristics made by the method disclosed in U.S. Pat. No. 4,872,984, wherein an amine salt is included in the aqueous aromatic polyamine solution employed in the interfacial reaction step.
An area of continuing interest and need is to provide reverse osmosis membranes having greatly improved flux while maintaining solute rejection characteristics. Advantages to be derived from development of such membranes include less expensive membrane process equipment and lower energy consumption due to lower fluid pressures and pumping requirements. Efforts toward this end have included variations in the amines and acyl halides used in the interfacial polymerization, and the usage of various processing and flux-inducing additives. For example, U.S. Pat. No. 4,643,829 discloses a change of the acyl halide reactant to a cyclohexane-based analog, which results in higher flux membranes, but with some decrease in salt rejection levels. U.S. Pat. No. 4,812,270 discloses high flux membranes made by post-treatment of aromatic polyamide membranes with phosphoric and tannic acids, but having significantly lower sodium chloride rejections. U.S. Pat. No. 4,983,291 discloses post-treatment of an interfacial aromatic polyamide membrane with citric acid to improve flux while maintaining salt rejection. U.S. Pat. No. 4,950,404 discloses use of polar aprotic solvent additives in the aqueous polyamine solution to improve flux of resulting interfacially formed membranes while maintaining good salt rejection, these polar aprotic solvents being characterized by having a capability to dissolve or plasticize the underlying porous substrate. U.S. Pat. No. 5,674,398 discloses interfacial aromatic polyamide reverse osmosis membranes with excellent flux prepared with the aid of a condensation polymerization catalyst, particularly a 4-dialkylaminopyridine catalyst, these membranes having a mostly flat, featureless surface with reduced propensity to surface fouling tendencies compared with similar membranes generated according to the preceding disclosures. U.S. Pat. No. 5,614,099 on the other hand, discloses use of 10-40% isopropyl alcohol in the aqueous polyamine solution to generate interfacially synthesized aromatic polyamide membranes with a high degree of surface roughness, thereby achieving enhanced surface area, which is said to result in increased membrane flux in separations. These and other various approaches serve to underline the continuing quest for improved compositions and processes to provide reverse osmosis membranes with superior combinations of flux and solute rejection characteristics. The present invention represents a new and previously unforeseen alternative to the aforementioned approaches.